Audiometer



Sept. 18, 1962 A. HYMAN 3,054,855

AUDIOMETER Filed June 10, 1957 2 Sheets-Sheet 1 Fig. 1

I N V EN TOR. Abraham H man A. HYMAN AUDIOMETER Sept. 18, 1962 2Sheets-Sheet 2 Filed June 10, 1957 INVENTOR. AZraZm A man BYW PatentedSept. 18, 1952 f u I rate 1 3,054,855 AUDIGMETER Abraham Hyman, QueensVillage, N.Y. (142 Claudy Lane, New Hyde Park, N .Y.) Filed June 10,1957, Ser. No. 664,863 4 Claims. (Cl. 1791) (Granted under Title 35, US.Code (1952), sec. 266) The invention described herein may bemanufactured and used by or for the Government for governmental purposeswithout the payment of any royalty thereon.

This invention relates to improvements in the art of audiometers andrelates more specifically to a portable audiometer of thesweep-frequency type which employs an electrical circuit having aphase-shift oscillator including a frequency determining phase shiftingnetwork, audio signal attenuator means, automatic impedance matchingnetwork for each step of audio frequency of output signal, and avariable resistive calibrating means all energized by a circuitcomprising a full wave rectifier joined by a double filter section to anoutput voltage regulation tube.

A satisfactory audiometer must enable the physician to accurately andsatisfactorily measure the hearing sensitivity and acuity of anindividual patient over the principal frequency range of audible sound.The frequency range which is important for hearing, and in which therange of hearing sensitivity is greatest lies between substantially 100and 8000 cycles per second, and an audiometer for determining hearingimpairments of deafened persons should be able to measure eighty percentloss of hearing for this range of frequency. In addition, a satisfactoryaudiometer should be able to measure steps of audible or sound frequencywithin the testing range above a pure tone and the intensity of the toneshould be accurately measurable. A change of sound intensity by onedecibel is substantially the minimum change detectible by the human ear.However, because of the uncertainty of judgment of an averageindividual, one decibel (unit of hearing loss) change in sound intensityis of no great significance. However, for most practical purposes intesting the hearing of a deafened individual, it is sufiicient to testthe individuals hearing with an audiometer capable of producing goodtones at frequency steps of 250, 500, 1000, 2,000, 3000, 4000, and 7000cycles per second. However, pure tones at frequency steps which are anoctave apart and extend over a range from 128 to 8192 cycles per secondand which are capable of determining ones hearing ability in steps offive decibels may be used if desired.

The improvement of this invention over the prior art resides in thedesign of the new circuit configuration, the employment of a minimum ofconventional tube circuit components, employment of a one tubephase-shift oscillator, means to vary the step frequency of theoscillator, automatic impedance matching means and means to quickly andreliably calibrate the device; a full-Wave rectifier power circuitproperly filtered and controlled by a linear type of voltage regulatortube, the output of which energizes a phase-shift oscillator which inturn is coupled by a low impedance matching transformer and networkselectively and variably coupled to a ladder type of sensitivityattenuator calibrated in decibels; capability of the device to be usedselectively with air conduction or bone conduction receiversincorporated in an economical, easy to manufacture, and portable hearingtesting device which produces reliable, stable, and critical end resultsin the examining and diagnosing of the impaired hearing of individualpatients by a physician.

It is a primary object of this invention to provide an audiometerembodying a phase-shift oscillator including a frequency determiningphase-shift network suitable for diagnostic and screening purposes whichmay be 2 quickly and accurately attenuated within an audio threshold ata predetermined frequency by a multiple choice of values in a fixedcalibrating circuit.

It is another object of this invention to provide a method of comparingthe relative hearing acuity of a patient before and after being fittedwith a hearing aid device under the same external conditions.

It is a still further object of this invention to provide an audiometerhaving excellent stability and wave shape.

It is another object of this invention to provide an audiometer ofdependable, simple, and efiicient design having a unidirectionalfull-wave rectification means, voltage control means, and a stable onetube variable oscillator means which may be light, portable, and easy tomanufacture at an economical cost.

It is a further object of this invention to provide a novel arrangementof network components and threshold level resistors on multi-deckswitches for efiicient manufacture and utilization of space.

It is another object of this invention to provide a low impedancetermination of the oscillator output circuit while the frequency of theoscillator is varied over a broad frequency range by termination ofganged voltage dividers of the attenuator at various frequency and audiothreshold levels without substantially varying the inductive couplingduring the attenuation of the phase-shift oscillator and the intensityof audio threshold in decibels at a given frequency.

The foregoing and other objects of the invention will be best understoodfrom the following description of an exemplification thereof, referencebeing bad to the accompanying drawings, wherein FIG. 1 is a perspectiveexterior view of the housing enclosing the circuit and control means ofinstant device; and

FIG. 2 is a circuit diagram of the audiometer.

FIG. 1 shows a suitable housing for the apparatus comprising a portablecabinet 20 having a front panel 21 on which are mounted a power switch22 pilot or telltale light assembly 23, frequency step control dial 24,audio signal attenuator or intensity dial 25 graduated in decibels,oscillator energizing switch 26 and selective earphone switch 27.

FIG. 2 illustrates the novel circuitry configuration of the presentinvention which is energized by an A.-C. plugin full-wave rectifierpower unit consisting of a shielded power transformer 12, lead andplug-in 83 and complementary rectifiers 15 and '16; filter circuitcomprising condensers 18 and 19, resistors 28 and 29 are operablyconnected by leads 32 and 84 to voltage regulator tube 30 and ground 31.The primary winding of power transformer 12 consists of inductivecoupling 12a with proper shielding and grounding as shown.

Member 11 denotes a safety fuse and member 22 an onand-olf switch orcircuit closing device. Rectifiers 15 and 16 and center-tapped lead 17are operably connected to secondary inductive coupling 13, as shown, toobtain a fullwave form of current supply. Rectifiers 15 and 16 may be ofthe selenium or other suitable type which require no filament voltage toenergize them, are economical in cost, and occupy little space.Secondary winding or inductive coupling 13a furnishes A.-C. current toenergize pilot or telltale light 14 and furnish filament voltage topentode tube 47. The current and output voltage of the power unit isunidirectional containing a minimum of ripple voltage approximating adependable A.-C. current supply with a substantial constant voltagepotential to the phase-shift oscillator network A which is inductivelycoupled by transformer 35 to resistors 36 and 37 and connected byvariable switch means 25 through resisbor 39 to signal attenuatornetwork B and conductive receivers or earphones 81 and 82. The output ofVR (voltage regulator) tube 30 is operably connected by lead 34, asshown, through switch means 26 and resistor means 40 to lead 41 andprimary winding means of shielded signal output transformer 35 connectedby lead 83 to anode 51 of pentode tube 47 as plate voltage. The fixedbias of cathode 48 of tube 47 is obtained by common lead connection 46connecting in parallel circuit arrangement resistance 44 and condenser45 between electrode means 48 and ground connection 42. The frequency ofaudio signal output of oscillator A is varied in steps of fre quency bycondensers C C and C and resistors R R and R having selected values asgiven in the following table:

It is to be understood that this invention is not limited to thefrequencies shown in the above table, as it is Well within the abilityof one skilled in the art to practice this invention by using otherdesirable frequencies. Dotted block diagrams a, b, c, d, e, f and g, asshown, illustrate a resistance capacitance phase-shifting networkconnected between the output and the input of the tube 47. The networkis so proportioned that the feedback between the plate and gridterminals is in phase at the frequency of oscillation desired. Anadvantage of using this type of oscillator is that it is economical tomanufacture, uses only one electronic tube, and results in a portableand compact instrument. When the gain of the pentode is adjusted eithermanually or by an automatic amplitudecontrol circuit, barely to maintainproper oscillations (not overloaded), almost a pure sine-wave outputaudio signal is obtained with good frequency stability. The phaseshiftoscillator means A is particularly stable and suitable for operation atfixed frequency steps and probably represents the simplest satisfactoryaudio oscillator for use with an audiometer. The circuitry orientationof block diagrams b, c, d, e, f and g is as shown in enlarged blockdiagram a, which shows the resistance capacitance phaseshifting networkcomprising resistors R R and R and condensors C C and C Each step offrequency of oscillator A has a separate network as designated by dottedblock diagrams a, b, c, d, e, 1 and g in which each block diagram isselectively connected by manually operated and mechanically gangedswitches as multi-decked switches comprising switch means 24a and 2412connected to plate 51 and grid 49 of tube 47, as shown. Grid 50 of tube47 is operably connected by condenser 43 to ground terminal point 42.Electronic tubes 30 and 47 are shielded to attenuate or eliminateradiation, hum, and noise pickup arising from causes external to thetubes which act either upon the tubes or upon the components of thecircuit. The output of oscillator network A is inductively coupled (toinsure safety of patient from current shock) by transformer 35 tovoltage divider and impedance matching circuit C comprising groundconnection 38, resistances 36-, 37, 39, attenuator B and R Each value ofvariable resistance R, is manually preselected by switch 240mechanically ganged to switches 24a and 24b. Output transformer 35inductively couples electrically the output signal of tube 47 tovariable switch means 25 which may be selectively connected to thevarious sections of attenuator network B to selectively vary in decibelunits the intensity of the audio signal to switch 27 which selectivelyconnects the audio signal to receivers 81 and 82 during an auditory testfor impairment of human hearing.

The circuit of this invention has a variable low impedance terminationto effect broad-band response in audio signal output of transformer 35to obtain optimum impedance match of output transformer 35 to attenuatorB. Transformer 35 may be of a shielded inexpensive type. Variableresistances R and R are sub-miniature potentiometers and provide meansfor accurate and rapid calibration of frequency and threshold audiosignal levels whereby an optimum impedance match of signal betweennetworks A, B, and resistors 36, 37 and 39 is reflected to plate 51 ofoscillator tube 47 for each step of frequency of the audio frequencysignal.

Resistances 36 and 37 are part of a voltage divider and impedancematching network constituting resistances 36, 37, 39, R and attenuatorB. R; consists of a plurality of step resistances, each of which are asub-miniature potentiometer. R is present in the calibration procedureto establish the threshold level for each frequency and manually variedby switch 240 for proper impedance match of coupling of oscillator A toattenuator network B for each frequency step of signal output ofoscillator A by switch 24c being mechanically ganged to switch means 24aand 2417 as multideck switch 24 to insure that R, is preset in thecalibration procedure to establish the threshold level for eachfrequency and manually and selectively varied for each proper thresholdfrequency level of signal without afiecting the matching of transformer35 in the circuit. The otentiometers R act as isolators between the loadresistors 36, 37 and the attenuator network B and maintain stability ofthe oscillations as well as establishing levels above threshold ofhearing.

In the practice of this invention resistances 54, 55, 56, 57, 58, 59,60, 61, 62, 63, 64, 65, 66 and 67 may have an ohmic value of 5.1 ohms;resistances 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79 and 80 mayhave an ohmic value of 15 ohms; condenser 43 has a capacitor value of 4mfd.; resistance 44 has a resistive value of 220 ohms; condenser 45 hasa capacitator value of 25 mfd. with a working voltage of 25 volts;resistor 36 has an ohmic value of 4.7 ohms; resistor 37 has an ohmicvalue of 4.7 ohms; resistor 39 may have an ohmic value of 2700 ohms;condensers 18 and 19 may each have a capacitator value of 40 mfd. with aworking voltage of 450 volts; resistance 28 may have an ohmic value of750 ohms with a 10 watt output rating; resistance 29 may have an ohmicvalue of 3000 ohms with a 10 watt output rating; tube 30 may be a VR(voltage regulator) tube of type 0A2 and pentode tube 47 may be a beampower amplifier tube of type 6005. However, it is to be understood thatthis invention is not restricted to the above values as it is wellwithin the ability of one skilled in the art to use other values withoutdeparting from the spirit and scope of the invention. Audio signalreceivers 81 and 82 may be either of the bone conducting or airconducting type. Switches 24a, 24b and 240 are mechanically gangedtogether for manual operation with dial 24 shown in FIG. 1. Theaforementioned impedance matching circuit and rectifier circuit areshielded and grounded, as shown in FIG. 2, to reduce the harmful effectof external hum or interference in audio receivers 81 and 82.

While there has been described what is at present considered to be thepreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention.

Having thus described my invention, what I claim as new and wish tosecure by Letters Patent is:

1. An audiometer comprising energizing means, phase shift oscillatormeans connected to the output of said energizing means and including aplurality of frequency determining network circuits, each said networkcircuit having a variable adjusting means and variable switch means forselectively connecting a frequency determining network to saidoscillator, audio transformer means connected to the oscillator means, aplurality of impedance matching network means selectively connected tothe output of said transformer means including variable resistiveadjusting means, a voltage divider means, and step attenuating resistivenetwork means, switch means intercon meeting said frequency determiningand impedance matching networks to selectively connect the necessaryimpedance matching network with a selected frequency determiningnetwork, said voltage divider means including series circuit means forinterconnecting said attenuator means to said transformer output forsubstantially isolating varying attenuator load reflections from saidoscillator transformer output, and earphone means connected to theoutput of said attenuation means for determining the hearing frequencyof partially deaf individuals.

2. An audiometer for determining the hearing impairments of personnelcomprising an energizing circuit, a phase-shift oscillator circuitincluding a separate frequency determining circuit for each frequencyand having an audio signal output, said oscillator circuit beingselectively and operably connected to the output of said energizingcircuit, variable impedance matching means inductively coupled bytransformer output means to the output of said oscillator circuit,adjusting means included in each frequency determining circuit for theoscillator circuit, said variable impedance matching means cooperatingby mechanical coupling with said adjusting means of said oscillatorcircuit, a plurality of conductive earphone receivers, variableresistive type attenuation circuit means connecting said variableimpedance means to said receivers, voltage divider means includingseries circuit means for interconnecting said attenuator circuit meansto said transformer output for substantially isolating varyingattenuator load reflections from said oscillator circuit and switchmeans selectively cooperating with said attenuator means to vary theintensity of said audio signal to said conductive earphone receivers.

3. An audiometer comprising an energizing circuit, phase-shiftoscillator means including pentode tube means with RC network circuitmeans providing a plurality of variable frequency steps connectedbetween the input electrode means and output electrode means of saidpentode tube means, said network permitting energy feedback between theanode and gride terminals of said tube, said oscillation circuit beingselectively and operably connected to the output of said energizingcircuit, variable impedance matching means inductively coupled bytransformer output means to the output of said oscillator circuit, afirst adjusting means included in said RC network for adjusting theoscillator circuit, a second adjusting means in the impedance matchingcircuit being mechanically and selectively coupled to first adjustingmeans included in said RC network of said oscillator circuit, aplurality of conductive earphone receivers having selective switchmeans, variable resistive attenuation circuit means, voltage dividermeans including series circuit means for interconnecting said attenuatorcircuit means to said transformer output for substantially isolatingvarying attenuator load reflections from said oscillator circuit, saidattenuator circuit means including means for selectively connecting saidvariable impedance matching means to said receivers to vary theintensity of said audio output signal in decibel units to saidconductive earphone receivers.

4. An audiometer comprising an energizing circuit, a phase-shiftoscillator including pentode tube means hav ing connected theretoresistive-capacitive network circuit means comprising a plurality offrequency determining circuits selectively connected between the cathodeelectrode means and anode electrode means of said pentode tube means,said network permitting energy feedback be tween anode and gridterminals of said tube means in a predetermined phase relationship atthe frequency of oscillation, said oscillator circuit being selectivelyand operably connected to the output of said energizing circuit, a firstvariable resistive adjusting means included in each of said frequencydetermining circuits selectively connected to said oscillator circuit,variable impedance matching network circuit means including a secondvariable resistive adjusting means, audio transformer coupled outputmeans inductively connecting said impedance matching network to theoutput of said oscillator circuit means, variable resistance attenuationcircuit means, voltage divider means including series circuit means forinterconnecting said attenuator circuit means to said transformercoupled output means for substantially isolating varying attenuator loadreflections from said oscillator circuit, a plurality of conductiveearphone receivers in cluding selective switch means, said variable:resistive attenuation circuit means including means for selectivelyconnecting said variable impedance network to said audio receiversincluding switching means whereby the intensity of said audio signal isselectively varied in decibel units to said receivers.

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